To mulch or not to mulch? It shouldn’t even be a question.

There’s wood chip mulch peeking out of all of our landscape beds

One of the popular arguments against mulching landscape and garden soils is that mulch delays soil warming and thus retards plant growth. Given that a well-chosen mulch will moderate temperature extremes – both hot and cold – is this an argument supported with evidence? In today’s post, I’m reporting the data I collected in visiting various parts of my home landscape and gardens and measuring soil temperatures.

My trusty soil thermometer

For measurements, I used a soil thermometer placed at the same depth in every soil tested. This required movement of mulch if mulch was present, so that thermometers were inserted completely into the soil. These thermometers read the entire length of the probe, so readings represent the average temperature in the top 5” of soil. I took close-up photos of each of the areas tested. I took 5 measurements for each location.

Our evening temperatures have been near or below freezing, but the past several days have been sunny and the air temperatures are well into the 50F range. On March 17, it was 68F at 2 pm in the sun, though it was 27F that morning. The morning after (March 18), it was 35F.

There are several interesting trends to see on the box-and-whisker graph:

The variation of soil temperatures is most extreme in unprotected soils
  • Mulched raised beds have the most consistent temperatures, with no differences seen at any time or in any location measured.
  • Unmulched soil mounds have extreme changes, mirroring air temperatures.
  • Bare soil in beds under sunny conditions have extreme changes mirroring air temperatures, but not as great as that in raised beds. They are warmest during the day and coldest during the night.
  • Bare soil in beds under shaded conditions are the coldest soils during the day and even colder at night.
  • Soil under living mulch (turf) and beds with varying depths of wood chip are cooler during the day than bare soil in sunny conditions, but warmer at night.
  • Bare soil in beds that were newly mulched are much warmer than bare soils not near mulched areas.
  • The soil temperature under turf or in beds at least partially mulched did not change at night (data not shown on graph).

Extreme temperature swings can result in the death of germinating seeds, seedlings, expanding buds, and other tissues that aren’t cold hardy. This is especially true of tissues near the soil surface, where temperature are colder than they are at increased depths. Unprotected soil mounds show huge daily vacillations; comparative raised structures under mulch are cooler during the day but warmer at night. And bare soil in the shade is colder than any mulched soils. Consistency is important for young tissues, as they have few protections against environmental extremes.

What my little experiment demonstrates is what mulch research has consistently shown: appropriate mulch materials will moderate soil temperature extremes due to air temperature fluctuations. Just because a bare soil is 55F in the daytime doesn’t mean it won’t be 35F at night.

Published by

Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and a Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors "The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors Books: http://www.sustainablelandscapesandgardens.com

14 thoughts on “To mulch or not to mulch? It shouldn’t even be a question.”

    1. Last fall I spread 2-3” hardwood wood chips over my dry shade flower garden. This spring I’d like to add compost. Can add compost over the chips or should I push them aside first? I’m concerned about water penetration since my site is so dry and is on an incline.

      1. Why do you need to add compost? Have you done a soil test to find out how much organic matter your soil contains? Wood chips do create their own compost layer in any case. The worst thing you can do is create nutrient overloads with excessive organic matter. And you should maintain at least 4″ of chips to prevent weeds.

  1. Ms. Chalker – Scott, you are my Guru. But, the 4 plus inches of arborist chips I placed down in several beds (tomatoes) are (not full of, but) very weedy. Before I would knock them back with the trimmer, but I can’t now without sending the chips flying. Oddly the beds that are mulched with salt hay are relatively free of weeds. Oh well………………you’re still my go – to for good info.
    Side note: both Penn State University and “Gardener’s World” recommend cardboard layer mulch! The horror!

    1. What kind of weeds are you seeing? If they were there before you put down the 4″ of chips, they can come back, especially herbaceous perennials and biennials. The best thing to do is use many (8-12″ inches) of chips to eliminate existing weeds. Then you can use a 4″ to eliminate new weed seed establishment.

      Sorry for the hassle! Give this method another try.

      1. They look like dactylis glomerata, or agropyron repens……….not sure. When I pull them up by hand they have an obnoxiously long taproot…..which usually breaks off.
        Thanks for your reply, if you don’t mind – next I need to ask about adding sulfur to lower my soil pH. But I’ll save that for another time.
        You’re the best!
        Thank you.

  2. Hi Linda – I have a large expanse of buckthorn in many areas of our 3 acre property. We really don’t want to use the harsh chemicals to eradicate. Wondering if a thick layer of wood mulch of 8-10” deep would kill trimmed buckthorn stumps over time? These are definitely small stumps and lots of them!

    1. If you trim and mulch deeply, it will kill the least vigorous. If any re-emerge, my suggestion is to clear the mulch, recut, and then paint glyphosate directly onto the stump itself. Then replace the mulch. We’ve done this successsfully and the spot painting of glyphosate means only the target plant is affected.

  3. I recently had to teach a “Wake up your garden for spring” class at my nursery and spent a lot of time searching for all your articles on building beds with arborist wood chips! I think fall is better for building new beds but it was part of the class description so I quickly taled them through raised beds, solarization, sheet mulching, and using arborist wood chips, pointing out that even though many “garden writers” continue to promote lasagna gardening/sheet mulching,*based on current research* it was no longer recommended. By the way, just search online and that’s ALL that is recommended! Since you just debunked SuperThrive (yes! what a silly product) I thought maybe you could run an experiment to find out actually how long it takes for cardboard to decompose under the best conditions. Or maybe it’s been done already? Thanks – I’m a big fan of all of you!

    1. There’s quite a bit of research on using corrugated cardboard in compost piles. It takes months for it to break down, and keep in mind that these are optimal conditions: shredded cardboard, sufficient moisture, oxygen, and heat. By extrapolation an entire sheet of cardboard, buried where oxygen levels are low, and often in cool temperatures is going to take much longer to degrade.

Leave a Reply